As is known, there is often the danger of self-excitation of undesired oscillations in amplifier circuits comprising transistors with a high transit frequency. Undesired oscillations caused by the influence of unavoidable parasitic inductances and capacitances in cooperation with the amplifier transistor occur in a frequency range generally located substantially outside the operating frequency. The parasitic inductances can be traced back to the electrode leads and the parasitic capacitances to capacitances located in the semiconductor body (e.g. barrier layer capacitances) and also to external wiring capacitances. Both parasitic components are also referred to as parasitic reactances. While the operating frequency in a VHF amplifier is, for example, 100 MHz, the undesired parasitic oscillations occur in the upper VHF or UHF range. The undesired oscillations result in reception interference and spurious radiation.
The undesired oscillations occur, above all, when the amplifier transistor is used in a grounded base circuit. In an emitter base circuit, however, the undesired oscillations are less frequent or easier to avoid. The problems of self-excitation of oscillations is particularly great in integrated circuits since, e.g. relatively large values exist for the parasitic lead inductances. It is known to prevent the undesired parasitic oscillations by providing the leads with coverings of ferrite beads or by placing ferrite beads or resistors in lead paths to the amplifier transistor. The known solutions do, however, have the disadvantage that the characteristics of the amplifier circuit are negatively influenced in the operating frequency range, particlarly when relatively large lead inductances are caused by the integration, and, more particularly, by an increase in the inherent noise of the amplifier circuit. The relatively high cost of known solutions is a further disadvantage. Since no satisfactory solution has yet been found to the aforementioned problems, pre-amplifiers for VHF uses are still made in accordance with the conventional technique today.